DOC. 23 PROPAGATION OF LIGHT 385 is nothing that compels us to assume that the clocks U, which are situated in different gravitational potentials, must be conceived as going at the same rate. On the contrary, we must surely define the time in K in such a manner that the number of wave crests and troughs between S2 and S1 be independent of the absolute value of the time, because the process under consideration is stationary by its nature. If we did not satisfy this condition, we would arrive at a definition of time upon whose application time would enter explicitly the laws of nature, which would surely be unnatural and inexpedient. Thus, the clocks in S1 and S2 do not both give the "time" correctly. If we measure the time at S1 with the clock U, then we must measure the time at S2 with a clock that runs 1 + 9/c2 times slower than the clock U when compared with the latter at one and the same location. Because, when measured with such a clock, the frequency of the light ray considered above is, at its emission at S2, L 1 + I and is thus, according to (2a), equal to the frequency v1 of the same ray of light on its arrival at S1. From this follows a consequence of fundamental significance for this theory. Namely, if the velocity of light is measured at different places in the accelerated, gravitation-free system K' by means of identically constituted clocks U, the values obtained are the same everywhere. According to our basic assumption, the same holds also for K. But, according to what has just been said, we must use clocks of unlike constitution to measure time at points of different gravitational potential. To measure time at a point whose gravitational potential is $ relative to the coordinate origin, we must employ a clock which, when moved to the coordinate origin, runs (1 + O/c2) times slower than the clock with which time is measured at the coordinate origin. If c0 denotes the velocity of light at the coordinate origin, then the velocity of light c at a point with a gravitational potential $ will be given by the relation [10] (3) c = c1[1 + c2]. The principle of the constancy of the velocity of light does not hold in this theory in the formulation in which it is normally used as the basis of the ordinary theory of relativity. § 4. Bending of Light Rays in the Gravitational Field From the proposition just proved, that the velocity of light in the gravitational field is a function of place, one can easily deduce, via Huygens' principle, that light rays propagated across a gravitational field must undergo deflection. For let c be a plane